Electrolytic cells for the production of chemicals such as chlorine and caustic soda from aqueous solutions of NaCl generally include a carbon steel cell within which anodes and cathodes are arranged and surrounded by electrolyte. Normally several anodes or a networked grid-like anode is placed within the cell from the bottom and the cathode walls of the cell are connected by a similar grid-like network of fingers so that anodes and cathodes are relatively evenly and closely spaced throughout the electrolyte. Current is supplied to the cathodes via a conductor strap (usually copper) attached to the exterior of the steel cell walls. Current flows from the cathodes through the electrolyte to the anodes and is carried from the anodes by a conductor which insulatingly passes through the bottom of the cell.
Conventional methods of fastening the copper conductor strap to the cathode's steel outer wall include fillet welding, brazing, or silver soldering. Unfortunately, the copper-steel bond is difficult to create and maintain and at best there remain air gaps between the copper strap and the steel cell wall. At worst, fine cracks, fissures, or separations occur in the steel wall of the cell or in the solder joint between the copper and steel. A crack, fissure, or separation allows a small amount of electrolyte to invade the crack, fissure, or separation. The invading corrosive electrolyte may be brine or any other corrosive fluid which spills onto or collects on the exterior surface of the cell. The corrosive liquid begins to attack the cell wall at the small crack, fissure, or separation. As the corrosive attack progresses, the crack, fissure, or separation becomes larger and results in the invasion of progressively more and more corrosive electrolyte. The end result is an ever compounding corrosive attack.
Eventually, large quantities of iron oxide and other contaminants are generated between the copper conductor strap and the cell's steel outer wall. The buildup of rust and other contaminants causes the forcing of the copper conductor strap away from the cell's outer wall. The force of escaping electrolyte actually breaks the welded or brazed or soldered bond between the outer wall and the copper conductor strap. At that point, a very large crack or fissure or separation traps corrosive fluid between the copper conductive strap and the cell's outer steel wall. If this contagion of corrosive attack is unchecked, it will continue until the copper strap is completely separated from the cell wall and the cell wall begins to leak its electrolyte at a rate which renders the cell functionally inoperative.
In practice, cells are removed from a productive cell line for repair prior to such extensive damage. However, the cells are usually allowed to function until the corrosive attack has pushed the copper conductor strap away from the steel outer wall in several places. During the progression of this corrosive attack, the electrolytic cell becomes progressively less efficient as the cracks, fissures, or separations between the copper conductor strap and the cathode's steel outer wall grows larger. The resultant loss in efficiency is caused by several factors, primarily the loss of continuous and uniform electrical conductivity between the copper conductive strap and the cathode's outer cell wall. Under these circumstances, parts of the damaged cathodes carry more electrical current than others. This means that part of the cell is working less efficiently than others. Electrical energy is lost, and the cell produces less product per kilowatt hour as a result.
There have been improvements in the design of electrolytic cells, but most of these improvements relate to the design and placement of the anode rather than improvements to the cathode. For example, U.S. Pat. No. 3,591,483 to Loftfield et al (the complete disclosure of which is incorporated herein by reference) discloses
Diaphragm-Type Electrolytic Cells; Use of Dimensionally Stable Anodes" where the cell is provided with a metal base serving as a rigid support and conductor for anodes and which supports the cell itself but is insulated from the cell by a sheet of non-conductive material which also provides an hydraulic seal to prevent leakage of electrolyte through the bottom of the cell.
U.S. Pat. No. 3,873,437 to Pulver (the complete disclosure of which is incorporated herein by reference) discloses an "Electrode Assembly for Multipolar Electrolytic Cells" where an anode carried by a lateral surface of a compartment wall is movable in a direction towards an opposed cathode surface to maintain a narrow gap between the electrodes. A cathode carried by the opposed lateral surface of the same compartment wall is optionally provided with means for moving it in a direction towards the opposed anode surface.
U.S. Pat. No. 3,878,082 to Gokhale (the complete disclosure of which is incorporated herein by reference) discloses a "Diaphragm Cell Including Means for Retaining a Preformed Sheet Diaphragm Against a Cathode" where a preformed sheet material is used as a diaphragm through the use of elasto-polymeric retainers.
U.S. Pat. No. 3,883,415 to Kokubu et al (the complete disclosure of which is incorporated herein by reference) discloses a "Multiple Vertical Diaphragm Type Electrolytic Cell for Producing Caustic Soda" where a large number of unit cells are installed compactly in a cathode tank in electrically parallel connection, each cell having two anode plates interwelded by at least two conductive supporting rods which in turn are connected to outer bus bars. An iron mesh cathode frame lined with an asbestos diaphragm surrounds the anode plates. A corrosion resistant cap is mounted on the iron mesh cathode and a bottom dish is inserted thereinto. The upper part of the anode plates are secured by insulated set screws.
U.S. Pat. No. 3,960,697 to Engler et al (the complete disclosure of which is incorporated herein by reference) discloses a "Diaphragm Cell Having Uniform and Minimum Spacing Between Anodes and Cathodes" where a continuous net is provided between the anodes and the diaphragm to permit minimum and uniform anode-cathode spacing while preventing the diaphragm from adhering to the surface of the anodes.
U.S. Pat. No. 4,211,629 to Bess et al (the complete disclosure of which is incorporated herein by reference) discloses an "Anode and Base Assembly for Electrolytic Cells" where downwardly facing annular portions of the anodes are welded to a perforated metal cell base cover which seals electrolyte in the cell from the cell base eliminating corrosion in the cell base and anode risers.
These improvements, while worthy in their own right, do not address the problem discussed herein, namely, the connection of a conductor strap to the exterior wall of the cell.
In connection with the present invention, it is also noted that bismuth alloys are known to have very low-melting temperatures and low physical strength and have been used as low temperature melting solders for safety devices like sprinkler links, plugs in compressed gas tanks and in fire alarm devices. Bismuth is a heavy, coarse crystalline metal which expands when it solidifies. Water and antimony also expand on freezing, but bismuth expands much more than the former, namely 3.3% of its volume. When bismuth is alloyed with other metals, such as lead, tin, cadmium and indium, this expansion is modified according to the relative percentages of bismuth and other components present. As a general rule bismuth alloys of approximately 50 per cent bismuth exhibit little change of volume during solidification. Alloys containing more than this tend to expand during solidification and those containing less tend to shrink during solidification. After solidification, alloys containing both bismuth and lead in optimum proportions grow in the solid state many hours afterwards. Bismuth alloys that do not contain lead expand during solidification with negligible shrinkage while cooling to room temperature.